The present invention relates to a toner composition for electrophotography, and more particularly to an electrophotographic toner composition having excellent fixing property at a low temperature, offset resistance, blocking resistance and the like which contains a specific polyester resin as a binder.
Various electrophotographic methods are hitherto known. There is generally known a method in which a latent electrostatic image is formed on a photosensitive layer utilizing a photoconductive material by various means and is then developed with a toner, and after transferring the toner image to a support such as paper if necessary, the toner image is fixed by heating, applying a pressure or using a solvent.
In recent years, a high speed fixing property is required for the toner in order to increase the efficiency of the copying operation. Attempts such as using a thermoplastic resin having a lower softening point and being easily heat fusible as a toner binder to be used in a conventional oven-heat fixing method have been investigated to provide a toner composition satisfying the requirements. However, mere lowering of the softening point of a resin binder has the defect of causing agglomeration of toner particles, namely blocking phenomenon, during storage or use. Accordingly, as one system for high speed fixing, there has been utilized a fixing method using a heat roller having a good heat conductivity.
Since the heat roller system is higher in heat efficiency than the oven-heat fixing system, a toner can be fixed in a shortened period of time. However, sticking of the toner to the heat roller, namely the so-called offset phenemenon, occurs, since the toner is directly brought into contact with the heat roller. Accordingly, it has been desired to develop a resin capable of solving this problem. A styrene-acrylate copolymer and a styrenebutadiene copolymer are usually known as a resin binder for a toner. These vinyl copolymers are in general poor in offset resistance and fixing property at a low temperature.
One of other resin binders is a bisphenol type epoxy resin. The bisphenol type epoxy resin provides a toner having superior fixing property at a low temperature because of having a lower molecular weight as compared with the vinyl copolymer binder. But, it has a tendency to lower the offset resistance, because the melt viscosity of the resin is low.
Recently, various polyester resins are proposed as a resin binder used in a toner which is excellent in both offset resistance and fixing property at a low temperature. A polyester resin produced by employing an etherificated diphenol as a main component of alcohol has been investigated from the viewpoint of the blocking resistance, offset resistance, and the like.
However, according to the inventors' researches, the polyester resin is excellent in blocking resistance, but does not have sufficient offset resistance and fixing property, in particular, fixing property at a low temperature.
Accordingly, the present inventors have been examined to develop a polyester resin which is excellent in blocking resistance and offset resistance and has well balanced properties with fixing property at a low temperature.
It is an object of the present invention to provide a novel electrophotographic toner composition which can solve the problem that conventional techniques can not solve, that is, an electrophotographic toner composition which satisfies at a time well-balanced offset resistance, blocking resistance and fixing property at a low temperature which are required for an electrophotographic toner composition containing a polyester resin as a binder and, in particular, which is suitably used in high speed machines has not yet been obtained.
In general, it is supposed that the improvement of fixing property at a low temperature of the electrophotographic toner composition will be accomplished by lowering the glass transition point or the molecular weight of the binder. Accordingly, the inventors have attempted to improve the fixing property at a low temperature by using a generally well-known aliphatic dicarboxylic acid having 4 to 10 carbon atoms as a plastic component to lower the glass transition point of an obtained resin binder. As the result, the fixing property at a low temperature of a toner composition for electrophotography in which the above resin is employed as a binder is improved to a certain degree. However, the improvement of the fixing property at a low temperature has not yet been sufficiently satisfied and also, there is a tendency that offset resistance and blocking resistance are lowered.
Thereupon, the present inventors have investigated to improve various properties, in particular, fixing property at a low temperature of a toner composition containing a polyester resin for electrophotography aiming at a plastic component. As the results, they have eventually found the marvelous facts that in case of employing a specific polyester resin obtained by employing at least one long chain aliphatic dicarboxylic acid selected from the group consisting of a linear aliphatic dicarboxylic acid having 16 to 34 carbon atoms, a dimer acid and a dibasic acid having 21 carbon atoms instead of the above-mentioned aliphatic dicarboxylic acid in the same amount of the aliphatic dicarboxylic acid, it is observed few differences between the glass transition point of the obtained toner composition and that of the above-mentioned composition in which an aliphatic dicarboxylic acid having 4 to 10 carbon atoms is employed, that the fixing property of the obtained toner composition is remarkably improved and further, that offset resistance and blocking resistance of the obtained toner composition is very excellent. Further, they have found that in case of using a modified resin binder having a broader range of molecular weight with a rosin glycidyl ester and/or a monocarboxylic acid, a toner for electrophotography more excellent in fixing property at a low temperature can be provided. The present invention has been eventually accomplished on the basis of the above researches.
These and other objects of the present invention will become apparent from the description hereinafter.